A simple model called ECOPATH allows to understand the relationships between different trophic levels and to estimate the mean annual biomass and ecological production, based on input data that are usually available. This model is used to describe the food-web of a rather simple ecosystem: the pelagic area in part of Lake Tanganyka (Burundi). Biomasses and ecological productions are computed. It is shown that most of the ecological production of zooplankton-eater clupeids is consumed by a predator, Luciolates stappersii . Previous estimates of biomasses appear to be relevant and the sensibility of ECOPATH model to variations of the values of needed parameters, mainly P/B is assessed.